Method of reducing frictional resistance of a hull, and frictional resistance reducing vessel

ABSTRACT

In the present invention, perpendicular flows (Fa) along a running direction of a hull ( 10 ) are induced accompanying the operation of a vessel, and vortex structures in the vicinity of a submerged surface ( 11 ) of the hull are varied by the perpendicular flows. As a result, energy consumption during operation of the vessel can effectively be conserved by reducing the frictional resistance with a low level of energy consumption.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method of reducing thefrictional resistance of a hull, and a frictional resistance reducingvessel.

[0003] 2. Description of the Prior Art

[0004] Conventionally, many technologies for reducing the frictionalresistance between the hull and water have been proposed for the purposeof conserving energy consumed during the operation of vessels. Forexample, technologies for feeding gas into the water and generating alarge number of bubbles in the vicinity of the wall surface (submergedsurface) of the hull shell plate in order to reduce the frictionalresistance using the bubbles have been proposed by the present applicantand others as disclosed in Japanese Unexamined Patent Application, FirstPublication No. Sho 50-83992, Japanese Unexamined Patent Application,First Publication No. Sho 53-136289, Japanese Unexamined PatentApplication, First Publication No. Sho 60-139586, Japanese UnexaminedPatent Application, First Publication No. Sho 61-71290, Japanese UtilityModel Application, First Publication No. Sho 61-39691 and JapaneseUtility Model Application, First Publication No. Sho 61-128185.

[0005] In these technologies, air pressurized by a pump, blower, orother pressurization apparatus is blown into the water from a pluralityof holes or a porous plate provided in the hull.

[0006] However, in these methods of reducing the frictional resistanceusing bubbles, since energy is required to operate the pressurizingapparatus, the amount of energy conserved as a result of reduction offriction using the bubbles ends up being lost. At locations ofcomparatively large water depths, such as the bottom of the vessel, inparticular, it is necessary to pressurize the gas to a high pressurecorresponding to the hydrostatic pressure when blowing the gas into thewater, thereby resulting in the consumption of a large amount of energy.In addition, for the installation of the pressurizing apparatus in thehull, huge costs are incurred, such as equipment costs and installationcosts.

SUMMARY OF THE INVENTION

[0007] In the method of reducing the frictional resistance of a hull ofthe present invention, to achieve the above objects, perpendicular flowsare induced along the running direction of the hull accompanying theoperation of the vessel, and vortex structures are varied in thevicinity of a submerged surface of the hull by the perpendicular flows.

[0008] According to this method, since the operation of an apparatus forreducing the frictional resistance is not required, energy consumptionduring the operation can effectively be conserved by reducing thefrictional resistance to a low level.

[0009] In the method of reducing the frictional resistance of a hull,the perpendicular flows are induced by vortices which are generated byan object which is provided on the submerged surface, or the vortexstructures are varied by vortices which are generated by an object whichis provided on the submerged surface, for example.

[0010] In addition to the aforementioned method of reducing thefrictional resistance of a hull, a method comprising a step for forminga negative pressure region at a low pressure relative to a gaseous spacein the water accompanying the operation of the vessel, and inducing agas being led from the gaseous space to the negative pressure region,can be performed for the purpose of reducing the frictional resistanceby releasing bubbles onto a submerged surface of the hull.

[0011] According to this method, by employing a pressure gradient formedaround the hull, the gas can be introduced into the water with an energyconsumption less than for the case of pressurizing the gas, andtherefore, the frictional resistance of the hull can be effectivelyreduced.

[0012] In the method of reducing the frictional resistance of a hull,the negative pressure region is developed by a cyclic flow which isgenerated by a wing which is provided on the submerged surface, and theperpendicular flows are generated by vortices which are generated by thecyclic flow, for example.

[0013] Furthermore, in the method of reducing the frictional resistanceof a hull, an upward lift which acts on the hull may be generated by thewing.

[0014] The present invention also relates to a frictional resistancereducing vessel for reducing the frictional resistance between asubmerged surface of a hull and water. The frictional resistancereducing vessel comprises a vortex generator which generates vorticesfor inducing perpendicular flows along a running direction of the hullaccompanying the operation of the vessel.

[0015] Furthermore, the frictional resistance reducing vessel maycomprise a negative pressure forming portion which is provided on thesubmerged surface for forming a negative pressure region at a lowpressure relative to a gaseous space in the water; an outlet (airoutlet) which is provided behind the negative pressure forming portion;a flow path of which one end opens to a gaseous space and the other endopens to the water via the outlet; and an object which generatesvortices for inducing perpendicular flows along a running direction ofthe hull accompanying the operation of the vessel.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a drawing that explains one embodiment of a method ofreducing the frictional resistance of a hull, and a frictionalresistance reducing vessel, according to the present invention.

[0017]FIG. 2A is a side view showing one embodiment of a vortexgenerator according to the present invention.

[0018]FIG. 2B is a plan view showing one embodiment of a vortexgenerator according to the present invention.

[0019]FIG. 3A is a side view showing another embodiment of a vortexgenerator according to the present invention.

[0020]FIG. 3B is a side view showing another embodiment of a vortexgenerator according to the present invention.

[0021]FIG. 4 is a drawing that explains another embodiment of a methodof reducing the frictional resistance of the hull and a frictionalresistance reducing vessel, according to the present invention.

[0022]FIG. 5 is a drawing of a large ship in which the present inventionis applied FIG. 6 is a bottom view of the large ship taken along anarrow A in FIG. 5.

[0023]FIG. 7A is a drawing that explains another embodiment of a methodof reducing the frictional resistance of a hull, and a frictionalresistance reducing vessel, according to the present invention.

[0024]FIG. 7B is a drawing that explains another embodiment of a methodof reducing the frictional resistance of a hull, and a frictionalresistance reducing vessel, according to the present invention.

[0025]FIG. 8A is a perpendicular cross-sectional view showing oneembodiment of a bubble generator according to the present invention.

[0026]FIG. 8B is a plan view showing one embodiment of a bubblegenerator according to the present invention.

[0027]FIG. 9A is a side view showing another embodiment of a bubblegenerator according to the present invention.

[0028]FIG. 9B is a plan view showing another embodiment of a bubblegenerator according to the present invention.

[0029]FIG. 10 is a drawing of a large ship in which the presentinvention is applied FIG. 11 is a bottom view of the large ship takenalong an arrow A shown in FIG. 10.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] Preferred embodiments of the method of reducing the frictionalresistance of a hull, and the frictional resistance reducing vessel ofthe present invention will be explained in the following.

[0031]FIG. 1 is a drawing schematically showing a flow of water in thevicinity of a submerged surface (bottom) 11 of a hull. In the method ofreducing the frictional resistance of the present invention, when avessel operates, flows Fa, which are perpendicular to a travelingdirection Dv of a hull 10, are induced, and these perpendicular flows Fachange the structure of vortices in the vicinity of the submergedsurface of the hull.

[0032] The frictional resistance of the hull is greatly influenced bythe structure of the vortices in the vicinity of the submerged surface(wall surface) of the hull, especially, in the turbulent boundary layer.In the vortices in the turbulence, perpendicular vortices such ashair-pin vortices (horseshoe vortices) are considered to have a greatinfluence on the frictional resistance in the turbulence. In the presentinvention, a reduction of the frictional resistance is accomplished bychanging the structure of the vortices, especially changing thestructure of the perpendicular vortices in the vicinity of the wallsurface of the hull by inducing the aforementioned perpendicular flowsFa.

[0033] The aforementioned perpendicular flows Fa are induced byproviding an object on the submerged surface of the hull to generate newvortices (derivative vortices) SC in the water, for example. A vortexgenerator 20 shown in FIG. 1 is provided for generating such vortices,and it is preferable that vortices are generated which extend from thebow to the stem of the vessel and which have axes along the longitudinaldirection of the vessel, for example.

[0034] The vortex generator 20 is formed of a combination of a pluralityof parts having predetermined shapes. Furthermore, the shape of thevortex generator 20 is designed using various kinds of analysis such ascomputational fluid dynamics (CFD), so as to minimize the resistance(drag) against the flow of the water which is generated by the operationof the vessel and so as to generate vortices which have theaforementioned desirable structures by the flow of the water.

[0035] For example, as shown in FIGS. 2A and 2B, the vortex generator 20is composed of a first member 21 which is provided approximatelyparallel to the submerged surface 11 of the hull separated by a space,and a second member 22 which is provided between the first member 21 andthe submerged surface 11 of the hull. In this embodiment, the firstmember 21 is a plate-shaped member which has a circular or an ovaloutline and a predetermined thickness, and the second member 22 is apost-shaped member which has an approximately semicircularcross-section. Furthermore, the second member 22 is provided so that acurved surface 22 a thereof faces the bow (front side in the travelingdirection). In addition, a material, such as a corrosion-resistant metalor plastic that has surface corrosion resistance primarily with respectto sea water and is also resistant to adherence to the surface by marineorganisms is preferably used for the material of the first and secondmembers 21 and 22.

[0036] Returning to FIG. 1, the vortex generator 20 generates vorticesin the water by varying the flow of the water Fw along the submergedsurface (wall surface) 11 of the hull. That is, the water flows alongthe curved surface 22 a, and vortices SC are generated as the waterpasses the curved surface 22 a. The vortices SC extend along the sternso as to rotate around the axes parallel to the longitudinal directionof the vessel (or the axes gradually separate from the wall surface asthey approach the stern).

[0037] In this case, the vortices SC generated by the vortex generator20 induce the flows Fa which are perpendicular to the travelingdirection Dv of the hull 10, and with the vanishing of the perpendicularvortices such as the hair-pin vortices, in the boundary layer adjacentto the wall surface, and with the carrying of the momentum of theperpendicular vortices to the outside of the boundary layer, theseperpendicular flows Fa change the structures of the vortices whichinfluence the frictional resistance. Consequently, the momentum of thewater in the boundary layer is decreased, and the frictional resistanceof the hull is reduced.

[0038] In the method of reducing the resistance of a hull which utilizesthe momentum of water only, since the disadvantage resulting from thewater depth is unlikely to occur as it does in the method by dispersinggas in water, an effective reduction of the frictional resistance can beeasily performed. In addition, the directions and axes of the vorticeswhich are newly generated by the vortex generator are not limited tothose shown in FIG. 1, and other directions and axes may be employed aslong as they can effectively change the structures of the vortices whichinfluence the frictional resistance in the boundary layer. Therefore,the shape, position, and number of vortex generators are suitablydetermined in compliance with conditions such as the shape of the hull,the standard cruising velocity of the vessel, and the like.

[0039]FIGS. 3A and 3B show another embodiment of the vortex generator.

[0040] In the vortex generator 30, the aforementioned first and secondmembers are formed as wing forms. That is, the vortex generator 30 iscomposed of a first wing 31 which is provided approximately parallel tothe submerged surface 11 of the hull separated by a space, and secondwings 32, 33, which are provided between the first wing 31 and thesubmerged surface 11 of the hull for supporting the first wing 31.

[0041] An NACA wing form, an Ozibal wing form, or various other wingforms can be applied as the shapes of these wings 31, 32 and 33, and aredetermined in compliance with the shape and the standard cruisingvelocity of the vessel. Furthermore, the wings 31, 32 and 33 areprovided so that their front and back ends face the traveling directionDv of the hull 10, and the first wing 31 is provided so that itssurfaces face the upper and lower directions.

[0042] In the vortex generator 30, vortices (derivative vortices) SC aregenerated by cyclic flow Γ which are generated around the wings 31, 32and 33 as shown in FIG. 4. For example, a cyclic flow Γ(1) which flowstoward the stem along the surface of the wing 31 that faces a waterwaysurrounded by the wings 31, 32 and 33 and which flows toward the bowalong the opposite (outer) surface of the wing 31, is generated aroundthe wing 31. Similarly, cyclic flows Γ(2) and Γ(3) which flow toward thebow along the surface of the wing 31 that faces the waterway and whichflow toward the stem along the opposite (outer) surface of the wing 31,are generated around the wings 32, 33. Therefore, a plurality of voricesSC are generated in the water by these cyclic flows Γ(1), Γ(2) and Γ(3).Furthermore, in the vortex generator 30, since the vortices aregenerated by the wings, the resistance (drag) against the flow of wateris reduced, and an effective reduction of the frictional resistance canbe easily performed.

[0043] The aforementioned vortex generator can be applied to a largeship in comparison with other types of vessels in which the surface areaof the bottom is formed to be large relative to the side of the vessel.In FIG. 5, reference symbol M indicates a frictional resistance reducingvessel; 40 is a hull; 42 is a hull shell plate (submerged surface); 43is a propeller; 44 is a rudder; and 45 is a water surface (waterline).The frictional resistance reducing vessel M is a large ship in themanner of, for example, a Very Large Crude-oil Carrier (VLCC).Furthermore, the aforementioned vortex generators 30 (or the vortexgenerators 20) are provided on the bottom of the hull in the vicinity ofthe bow of the vessel.

[0044]FIG. 6 shows the bottom of the hull 40 in the vicinity of the bowof the vessel in which the vortex generators 30 (or the vortexgenerators 20) are provided. A plurality (here, two) of vortexgenerators 30 are arranged in a row along the width of the bottom of thevessel, and as a result, vortices can be generated in a wider area.

[0045] Next, another embodiment of the method of reducing the frictionalresistance of a hull and the frictional resistance reducing vessel ofthe present invention will be explained in the following.

[0046]FIGS. 7A and 7B schematically show the flow of water in thevicinity of the submerged surface (bottom) 11 of the vessel 10. FIG. 7Ais a cross-sectional view of the hull 10 as shown from the side of thehull, and FIG. 7B is a cross-sectional view of the bottom of the hull 10as shown from the water. In the method of reducing the frictionalresistance of the hull of the present embodiment, the method employs a“negative pressure method”, in which a negative pressure region 12 whichis at a low pressure relative to the atmosphere, in the water during theoperation in order to release gas into the water without using apressure device.

[0047] The negative pressure region 12 can be formed by varying therelative flow of the water during the operation of the vessel resultingfrom the occurrence of a separation region or cavitation formed by meansof an indentation 13 or an edge member 14 on the submerged surface 11 ofthe hull 10. At this time, by opening one end of a flow path 16 (airoutlet 17) to the negative pressure region 12 and the other end (airinlet 18) to the atmosphere, the force of a pressure gradient Pf(pressure in the air outlet 17 <pressure in the air inlet 18) acts onthe fluid in the flow path 16. In this embodiment, air which has flowedin the flow path 16 from the atmosphere through the air inlet 18 is fedinto the water through the air outlet 17 in the form of microbubbles 120by using the force of the pressure gradient Pf.

[0048] Here, when bubbles having a volume Qv are released at a locationat a depth h (m) from a liquid surface in a static liquid of density ρ(the density of the bubbles is assumed to be zero), the energy requiredfor their release is represented by the following equation:

E=(P−Pa)Qv  (1)

[0049] where Pa is the pressure of the gaseous space (atmosphericpressure), and P is the pressure at the location where the bubbles arereleased (=ρgh, where g is gravitational acceleration).

[0050] At this time, if the flow rate in the vicinity of the air outlet17 on the bottom is taken to be V₁, then the pressure P at that locationis represented by the following equation:

P=Pa−ρ(V ₁ ² −V ²)/2+ρgh  (2)

[0051] Furthermore, although the flow rate V₁ changes according to therelease of bubbles into the boundary layer, this change is ignored here.

[0052] As is clear from Equation (1), for the case when the pressure Pat the location where bubbles are released is low in comparison with theatmospheric pressure Pa, namely when P<Pa, the energy becomes negative(E<0), and additional energy is not required to move air to the bottom.That is, in the method of reducing the frictional resistance of the hullof the present invention which employs the negative pressure method,bubbles can be generated in the water using less energy thanconventional pressurization methods. Furthermore, in addition to thebubbles that move from the atmosphere to the bottom via the flow path16, the bubbles in the vicinity of the air outlet 17 also includebubbles generated when the pressure of the negative pressure region 12becomes low in comparison with the saturated vapor pressure as a resultof cavitation and separation that occur results from the pressure of theindentation 13 or edge member 14.

[0053]FIGS. 8A and 8B show an embodiment of a bubble generator forgenerating bubbles in the water. A bubble generator 50 comprises arecess 51 which is formed on the submerged surface 11 of the bottom ofthe hull, a flow path 16 in which one end opens to the atmosphere andthe other end opens to the recess 51, and a hood 52 which is provided onthe submerged surface 11 so as to cover at least a part of the recess 51from the underwater side.

[0054] In this embodiment, the recess 51 is formed by connecting amember 53 having a crooked plate shape with a rectangular-shaped openingbored through the submerged surface 11 of the hull, and the recess 51 isformed so that the depth from the submerged surface 11 gradually becomesshallower in the direction of the stem. In addition, the flow path 16 ofthis embodiment is provided as an inner space of an air induction pipe(AIP) 54 which is connected with the plate shaped member 53 for formingthe recess 51. That is, an opening 53 a for introducing a gas into thewater is bored through the plate-shaped member 53, and the tubular airinduction pipe (AIP) 54 is connected with the opening 53 a. Furthermore,the hood 52 is a member having a crooked plate shape, and an inclinedsurface 52 a is formed thereon so that the amount projecting from thesubmerged surface 11 is gradually increased in the direction of thestem, and a part of the inclined surface 52 a covers the recess 51 fromthe underwater side. Moreover, the bow end of the hood 52 is connectedwith the submerged surface 11, and the other end (stem end) of the hood52 is positioned at a predetermined height from the submerged surface 11of the hull.

[0055] In this bubble generator 50, the relative flow of the waterduring the operation of the vessel is varied by the inclined surface 52a of the hood 52 and the recess 51. For example, at the inclined surface52 a of the stem side edge member 14 a and at the indentation 13 on thetip end of the recess 51, cavitation and separation occur due to thesharp angle thereof, and hydrostatic pressure decreases. Furthermore,the inclined surface 52 a of the hood 52 facilitates a decrease inhydrostatic pressure, as a result of narrowing of the water flow pathand the increase in the water flow rate, since the amount of theinclined surface 52 a projecting from the submerged surface 11 isgradually increased in the direction of the stem. Consequently, theaforementioned negative pressure region is formed in the water.Moreover, some of the bubbles which are released from the flow path 16enter into the recess 51 and are diffused along the width of the vessel,and therefore, a large area of the submerged surface 11 of the hull iscovered by concentrated bubbles. In addition, as a result of cavitationand separation occurring at the inclined surface 52 a of the stem sideedge member 14 a and the indentation 13 on the tip end of the recess 51in the negative pressure region, the gas and water is actively mixed atthe interface thereof, facilitating the release of bubbles from theinterface. Furthermore, in the bubble generator 50, since the recess 51and the hood 52 are manufactured using a crooked plate shape member, aplurality of recesses 51 and hoods 52 can be provided on the submergedsurface 11 of the hull without reducing the strength thereof.

[0056] Returning to FIG. 7, in this embodiment, a wing 25, which has asurface approximately parallel to the submerged surface 11 of the hull,generates the cyclic flow Γ(1), which develops the negative pressureregion 12. That is, the cyclic flow Γ(1), which flows toward the stem(rear) along the surface of the wing 25 that faces the submerged surface11 and which flows toward the bow (traveling direction Dv) along theopposite surface of the wing 25, is generated around the wing 25. Atthis time, in the waterway between the submerged surface 11 of the hulland the wing 25, the flow rate thereof is increased because the momentumof the water caused by the cyclic flow Γ(1) is added to the flow 15 ofthe water along the submerged surface 11, and therefore, the pressure inthe negative pressure region 12 is further decreased. Consequently, thenegative pressure region 12 is further developed, the force of thepressure gradient Pf is increased, a large quantity of air bubbles 120are released into the water, and the frictional resistance of the hullis effectively reduced by the presence of these bubbles 120 on thesubmerged surface 11.

[0057] Furthermore, in this embodiment, due to the circulating flow Γ(1)that occurs around the wing 25 (because the flow rate over the upwardfacing wing surface is greater than that over the downward facing wingsurface), a pressure difference occurs above and below the wing 25, andan upward lift Lf acts on the hull 10 from the wing 25. Consequently,the bow of the hull 10, in particular, is raised up due to this lift,the submerged surface area of the hull 10 decreases, and the frictionalresistance of the hull 10 is further reduced.

[0058] Moreover, in this embodiment, similar to the embodiment shown inFIG. 1, the flows Fa, which are perpendicular to the traveling directionDv of the hull 10, are induced, and the perpendicular flows Fa changethe structure of the vortices in the vicinity of the submerged surface11 in order to reduce the frictional resistance of the hull 10 as shownin FIG. 7B.

[0059] That is, in this embodiment, the perpendicular flows Fa areinduced by providing objects 26, 27 on the submerged surface of the hullfor generating new vortices (derivative vortices) SC in the water, andthe perpendicular flows Fa change the structure of the vortices relatedto the frictional resistance, such as the perpendicular vortices in thevicinity of the submerged surface 11, in order to reduce the frictionalresistance of the hull 10, for example.

[0060] The vortices extending from the bow to the stern of the vesseland having axes along the longitudinal direction of the vessel, forexample, are preferably provided as the vortices SC, similar to those ofthe embodiment shown in FIG. 1. Furthermore, the vortices SC can beeasily generated by generating the circulating flow Γ(2), Γ(3) aroundthe objects 26, 27. The vortices SC extend along the stem and change soas to rotate around the axes parallel to the longitudinal direction ofthe vessel (or axes gradually separate from the wall surface as theyapproach the stem), and induce the flows Fa which are perpendicular tothe traveling direction Dv of the hull 10. These perpendicular flows Fachange the structures of the vortices related to the frictionalresistance, similar to those of the embodiment shown in FIG. 1.Consequently, the momentum of the water in the boundary layer isdecreased, and the frictional resistance of the hull is reduced.

[0061] Moreover, in this embodiment, the vortices (derivative voritices)SC generated by the objects 26, 27 are developed by mixing theaforementioned bubbles 120 into the vortices SC. That is, when thebubbles are mixed with the vortices SC, flows Fd of the fluid aregenerated along the direction of separation from the wall surface of thehull by means of the momentum of the bubbles caused by their buoyancy.Therefore, a plurality of bubbles are formed since the momentum of theoriginal vortices are diffused, the momentum of the vortices related tothe frictional resistance in the boundary layer is decreased, and thefrictional resistance of the hull is reduced. In this case, it ispreferable that the objects 26, 27 for generating the bubbles areprovided in front of the aforementioned air outlet 17 on the submergedsurface 11 so as to separate on both sides of the air inlet 17 along thewidth of the vessel, in order to mix the bubbles 120 into the vorticesSC effectively.

[0062]FIGS. 9A and 9B show an embodiment of a wing body 60 which has theaforementioned wing 25 and struts 26, 27 as the aforementioned objects.One end of each of the struts 26, 27 is connected with the submergedsurface 11 of the hull 10 and the other end of each of the struts 26, 27is connected with each end of the wing 25 along the width thereof. AnNACA wing form, an Ozibal wing form or various other wing forms can beapplied for the shapes of the wing 25 and struts 26, 27, and aredetermined in compliance with the shape and the standard cruisingvelocity of the vessel. Furthermore, the wing 25 and struts 26, 27 areprovided so that their front and back ends face the traveling directionDv of the hull 10, and the wing 25 is provided so that its convexsurface faces upward. In addition, a material, such as acorrosion-resistant metal or plastic that has surface corrosionresistance primarily with respect to sea water and which is alsoresistant to adherence to the surface by marine organisms, is preferablyused for the material of the wing 25 and struts 26, 27. Moreover, thedirections and axes of the vortices SC which are newly generated by thewing body 60 are not limited to this, and they may also be of anotherform as long as they can effectively change the structures of the vortexconcerning the frictional resistance in the boundary layer. Furthermore,the vortices SC can be generated with low resistance (drag) against theflow of water by employing the circulating flow generated by the wing.

[0063] The aforementioned bubble generator 50 and wing body 60 can alsobe applied to a large ship as shown in FIG. 10 similar to the vortexgenerators 20, 30, for example. In addition, FIG. 11 shows the bottom ofa hull 40 in the vicinity of the bow of the vessel in which the bubblegenerator 50 and the wing body 60 are provided. A plurality (here, two)of pairs of the bubble generator 50 and the wing body 60 are arrangedalong the width of the bottom of the vessel. Since the pairs arearranged along the width, a wider area on the bottom can be covered bythe vortices and the frictional resistance of the hull can be reducedmore effectively. Furthermore, the size, number and location of thebubble generators and wing bodies are suitably determined in compliancewith conditions such as the shape of the hull and the standard cruisingvelocity of the vessel. Moreover, the shape of the bubble generator andwing body are suitably determined based on the results of flow fieldanalysis, operational testing, etc. obtained by computational fluiddynamics (CFD), so that the resistance (drag) against the flow of wateris minimized, and the flow of water is in the desired state duringoperation.

[0064] In this embodiment, the frictional resistance of the hull isreduced more effectively as a result of the generation of circulatingflows Γ(1), Γ(2) and Γ(3) by the wing body 60. Since adequatecirculating flows are generated even during low-speed operation (forexample, at about 10 knots), this embodiment can be applied over a widerange of operating velocities.

[0065] Furthermore, since bubbles which are mixed into the water areformed at an internal pressure lower than the hydrostatic pressurecorresponding to the water depth, when the above bubbles move at aconstant water depth (for example, when the bubbles are moving along thevessel bottom), a higher water pressure acts on the bubbles the fartherthey move away from the negative pressure region, thereby causing thesize of the bubbles to gradually become smaller. According to researchconducted thus far by the present applicants, comparatively smallbubbles are preferable for reducing the frictional resistance of hull.Thus, bubbles generated by the negative pressure also act advantageouslyfor reducing the frictional resistance with respect to this point aswell.

[0066] In addition, the various shapes and combinations, etc. of eachcomposite member shown in the embodiment described above refer to only asingle example, and can be altered in various ways based on designrequirements and the like within a range that does not deviate from thepurpose of the present invention. Furthermore, since the presentinvention is not considered to be susceptible to disadvantages caused bywater depth, the frictional resistance reducing vessel of the presentinvention is also advantageous for application to the aforementionedlarge ships. However, the type of vessel to which the present inventionis applied is not limited to large ships, and the hull may also be ofanother form such as that of a high-speed vessel or a fishing vessel.

What is claimed is:
 1. A method of reducing frictional resistance of ahull comprising a step for inducing perpendicular flows along a runningdirection of the hull accompanying operation of a vessel, and a step forvarying vortex structures in the vicinity of a submerged surface of thehull by the perpendicular flows.
 2. A method of reducing frictionalresistance of a hull according to claim 1, wherein said perpendicularflows are induced by vortices which are generated by an object which isprovided on said submerged surface.
 3. A method of reducing frictionalresistance of a hull according to claim 1, wherein said vortexstructures are varied by vortices which are generated by an object whichis provided on said submerged surface.
 4. A frictional resistancereducing vessel for reducing frictional resistance between a submergedsurface of a hull and water, comprising a vortex generator whichgenerates vortices for inducing perpendicular flows along a runningdirection of the hull accompanying operation of the vessel.
 5. A methodof reducing frictional resistance of a hull by releasing bubbles onto asubmerged surface of the hull comprising: a step for forming a negativepressure region at a low pressure relative to a gaseous space in thewater accompanying operation of a vessel, and inducing a gas being ledfrom the gaseous space to the negative pressure region; and a step forinducing perpendicular flows along a running direction of the hull andvarying vortex structures in the vicinity of the submerged surface bythe perpendicular flows.
 6. A method of reducing frictional resistanceof a hull according to claim 5, wherein said negative pressure region isdeveloped by a cyclic flow which is generated by a wing which isprovided on said submerged surface, and said perpendicular flows aregenerated by vortices which are generated by the cyclic flow.
 7. Amethod of reducing frictional resistance of a hull according to claim 6,wherein an upward lift which acts on the hull is generated by said wing.8. A frictional resistance reducing vessel for reducing frictionalresistance between a submerged surface of a hull and water, comprising:a negative pressure forming portion which is provided on the submergedsurface and which forms a negative pressure region at a low pressurerelative to a gaseous space in the water; an outlet which is providedbehind the negative pressure forming portion; a flow path of which oneend opens to a gaseous space and the other end opens to the water viasaid outlet; and an object which generates vortices for inducingperpendicular flows along a running direction of the hull accompanyingoperation of the vessel.